1. Field of the Invention
The present invention relates to a beam irradiation device suitably used for, for example, vehicle-to-vehicle distance detectors, distance detectors, and the like.
2. Description of the Related Art
In recent years, vehicle-to-vehicle distance detectors and distance detectors using laser beams have been employed in various devices. A vehicle-to-vehicle distance detector, for example, detects the presence or absence of an obstacle and measures a distance to the obstacle by detecting reflected beams that are obtained when laser beams are radiated from a front portion of a vehicle. In this case, the laser beams are caused to longitudinally and laterally scan a target region preset in a front space. A time difference between timing for radiating the laser beams and timing for receiving the reflected beams is measured at each scan position, and a distance to an obstacle located in front of each scan position is calculated from a result of the measurement.
These detectors employ a so-called beam irradiation device that irradiates a target region with laser beams while longitudinally and laterally scanning the target region. This beam irradiation device is provided with, in order to protect an interior thereof from an external environment, a transparent window (output window) at a laser beam window. This output window is constituted from such as a glass plate or a resin plate.
However, in a case where dirt, water droplets, and the like are attached to, or such as a flaw or a breakage occurs to this output window, power of the laser beams attenuates when passing through these potions of the output window. As a result, measurement performance declines. Such a decline in measurement becomes a serious problem when the beam irradiation device is for an in-vehicle application. Therefore, especially when the beam irradiation device is used in-vehicle, it is necessary to configure the beam irradiation device to irradiate the laser beams uniformly over an entirety of the target region.
Japanese Patent Application Laid-Open No. 2005-10094 discloses a technique for detecting an attachment of dirt to the output window as described below. According to this technique, the presence of dirt on the output window is determined based on a time period between timing for radiating of the laser beams and timing for receiving of the reflected beams as well as an intensity of the reflected beams. Specifically, the laser beams are irradiated for a plurality of times, and laser beams with a time period from irradiation to reception of the reflected beams shorter than a threshold value and an intensity of the reflected beams is greater than a threshold value are detected from the laser beams irradiated for the plurality of times. Then, it is determined that dirt is attached to the output window, when the number of the detection of such laser beams is equal to or greater than a predetermined number.
However, with this conventional technique, when the laser beams irradiated from a semiconductor laser are reflected on the dirt attached to the output window into the beam irradiation device, the reflected beams leak to a beam receiving portion for receiving reflected beams from the target region. The leaked beams appear as noise in beam reception signals during a normal scan operation, and, as a result, cause such a problem of deterioration in accuracy in vehicle-to-vehicle distance detection and distance detection.